A conventional type of automotive radiator is described in Japanese patent laid-open publication 55-41391 and 58-1729419 and has a coil spring for biasing a pressure valve in order to close the valve. The pressure valve of the conventional type of the automotive radiator cap, however, has a disadvantage that the pressure valve cannot smoothly sit on a seal portion.
The reason why the pressure valve of the conventional type radiator cap cannot sit smoothly is explained hereinafter with reference to FIG. 5. Numeral 501 in FIG. 5 shows a top surface of a filler neck, numeral 503 shows a pressure valve and letter F.sub.0 shows a closing force of the coil spring.
Since the coil spring is contacted with the pressure valve 503 at the portion which is higher than the top surface 501 of the filler neck, and since the closing force F.sub.0 forces a top surface 505 of the pressure valve 503 vertically, the force F.sub.1 which has an orientation parallel with the top surface 505 of the pressure valve 503 occurs when the pressure valve 503 is inclined as shown in FIG. 5. The force F.sub.1 is calculated as next formula EQU F.sub.1 =F.sub.0 .times.sin .theta.
wherein .theta. means opening angle of the pressure valve 503 and D shows the distance between the top surface 505 of the pressure valve 503 and a sealing portion 507 of the pressure valve 503. The force F.sub.1 causes a rotating moment M.sub.0 which is calculated with the next formula EQU M.sub.0 =F.sub.1 .times.D.
Therefore the moment M.sub.0 causes the pressure valve 503 to rotate clockwise as shown by arrow R in FIG. 5.
Therefore, the force F.sub.1 makes the pressure valve 503 open even though the coil spring biases the pressure valve 503 to the closing position. Accordingly, the force F.sub.1 makes it hard for the pressure valve to sit smoothly on the filler neck 501.
Another type of the conventional automotive radiator cap is described in U.S. Pat. No. 3,265,048 and has also the coil spring biasing the pressure valve toward the filler neck in order to open the filler neck when the pressure within a radiator tank increases to a predetermined pressure. Since the pressure valve increases the boiling temperature point of the coolant within the radiator tank, the pressure valve does not open the filler neck when the temperature of the coolant within the radiator tank increases to its boiling point under normal atmospheric pressure. The pressure valve opens the filler neck when the temperature of the coolant increases to greater than the boiling point under normal atmospheric pressure and the pressure of the coolant increases to greater than a predetermined value.
The pressure valve has a connecting hole at the center portion thereof and has a pressure modulating valve provided within the connecting hole so that the pressure modulating valve opens or shuts the connecting hole.
The pressure modulating valve normally opens the connecting hole by its weight so that the pressure within the radiator tank can be maintained under the normal atmospheric pressure during the condition when the pressure within the radiator tank is under the predetermined pressure under which the coolant is boiling. When the coolant within the radiator tank vaporizes and the pressure of the steam increases up to the predetermined pressure, the pressure of the steam moves the pressure modulating valve upwardly so that the pressure modulating valve shuts the connecting hole.
The pressure within the radiator tank normally increases gradually due to the operating period of the automotive engine. The pressure within the radiator tank, however, increases rapidly under special circumstances such as the engine is racing.
Under such special circumstances the conventional radiator cap has a serious disadvantage, namely the pressure modulating valve cannot open the connecting hole after the pressure modulating valve is closed by the rapidly increasing pressure. Since the pressure within radiator tank is normally under air pressure, the pressure modulating valve is forced to its closing position by the normal pressure within the radiator tank after the pressure modulating valve is closed by the rapidly increasing pressure within the radiator tank.
Accordingly, the rapidly increasing pressure is held within the radiator tank by the pressure modulating valve, and such pressure can cause some pressure damage to the tube.
Another type of conventional automotive radiator has a cap as shown in FIG. 12. This conventional cap has a pressure valve body 804 which seals a filler neck and a pressure modulating valve 806 which opens and shuts the connecting hold 805 provided in the pressure valve body 804. Since the pressure modulating valve 806 is biased by a spring 808 in order to close the connecting hole 805, the pressure modulating valve cannot be operated frequently and the travelling distance of the pressure modulating valve when the pressure modulating valve opens the connecting hole is slight.
Therefore, this conventional pressure modulating valve has the disadvantage that objects located between the pressure modulating valve 806 and the connecting hole 805 are difficult to remove after such objects are adhered between the pressure modulating valve 806 and the connecting hole 805.